In manual mode when taking a burst of images.......the first image is always a stop or so brighter than the rest. Also, the 2nd and onward images are about a meg and a half smaller. After the first image all the others maintain the same exposure and image size as would be expected in maul mode.

So.......what do you think the issue is? (It's currently on the way to CPS, per their request)

Look at the exif for the images. Are the reported shutter speeds, apertures, and ISO remaining the same for the first and subsequent shots? If they are, it might be something goofy in the amp. Either way, I'm sure CPS will sort it out, but it's interesting to speculate.

So was the first shot too bright or were the others too dark ?
How did the first shot compare with taking a single shot ?

It'll be interesting to hear what CPS find but I wonder if it was a lens aperture diaphragm problem rather than a camera problem. The answers to the questions above might steer you to the lens unless CPS find something wrong with the camera.

The changing file size is not a worry - if the image is darker then there is perhaps less data and it is easier to compress the file. Conversely a brighter but not blown out image might have more data and be harder to compress.